It is often possible to model the body burden of pollutants as a mixed exponential moving average of the external pollutant dosage. This model fails: (a) when an appreciable fraction of pollutant is retained in long-lived components e.g. bone-seekers such as lead. A semi-Markov model for physiological kinetics generalizes this model. By representing body burden as a stochastic integral of external dosage, it is often possible to estimate the frequency, duration, and parameters of other integral exceedance measures for body burden, thus to estimate the effects of environmental quality standards on body burden variation. Physiological kinetic models for carbon monoxide, for the lead-calcium system, and for cadmium-zinc, will be estimated using non-linear regression techniques for experiments (as reported in the literature) on uptake and elimination in constant environments. Stochastic models will be developed for external dosage of CO and lead, and the parameters of the exceedance measures of body burden estimated. In this way it may prove possible to establish a better rationale for environmental standards setting.